1. Field of the Invention
The invention relates to a magnetic resonance spectroscopy method where an examination zone is subjected to a plurality of sequences, each sequence comprising at least two rf pulses, the distance in time between the rf pulses or their phase or their amplitude being varied from one sequence to another.
2. Description of the Related Art
A method of this kind is known inter alia from J. Chem. Phys., Vol. 64, No. 10, May 15, 1976, pages 4226 to 4227. Therein, each sequence consists of a 90.degree. rf pulse and a 180.degree. rf pulse whose distance in time is varied from one sequence to another. From the echo signals occurring subsequent to the 180.degree. rf pulses there is derived a two-dimensional Fourier spectrum, the transformation parameter for one frequency axis being the variation in time of the signals whilst that for the other frequency axis is formed by the distance in time.
Also known are spectroscopy methods where the composition of the sequences is different and/or where, instead of the distance in time of the rf pulses, their amplitude or phase position is varied from one sequence to another. These methods also enable a two-dimensional spectrum to be obtained by means of two-dimensional Fourier transformation.
On the other hand, sequences of the kind set forth are also suitable for performing so-called multiple quantum experiments where higher quantum coherences are excited by the rf pulses.
In medicine there is a need for spatial resolution spectroscopy (Tropical Magnetic Resonance) where only a strictly defined examination zone (for example, a complete organ) is subjected to spectroscopic analysis. To this end it is known (J. Mag. Reson. 56, pages 350 to 354 (1984)) to select the volume to be excited before the spectroscopic excitation by sustaining the nuclear magnetization within the region of interest, the nuclear magnetization being rotated through 90.degree. and subsequently dephased outside this region, so that it can no longer make a signal contribution. The region to be selected is defined in that three mutually perpendicular layers are excited, each layer involving a selective 45.degree. pulse, a 90.degree. wideband pulse and a further 45.degree. rf pulse in the presence of a magnetic gradient field. The spectroscopic excitation is subsequently performed by means of a single rf pulse.
This known method for volume selection, however, influences the subsequent spectroscopic excitation so that it is not suitable for two dimensional spectroscopy, and notably not for multiple quantum experiments.